The invention relates to a method of desulfurizing a molten metal in a cupola and more particularly to a method of desulfurizing iron in a plasma fired cupola.
The properties of iron depend on the levels of critical constituents that are present in the iron in the solution phase. One important constituent that greatly effects the properties is sulfur, S. In most grey iron products the percentage of sulfur in the composition of the iron is kept below 0.1%. In modular or ductile iron the percentage of sulfur is in the range of 0.008%.
When scrap iron is remelted, sulfur, mainly from the coke enters the melt and has to be removed prior to casting the iron. Removal of sulfur from iron has been accomplished in the cupola by what is called the basic slag mode, which requires that a high portion of the charge includes a slag forming material such as dolomite or limestone. The dolomite or limestone makes up about 5% to 15% of the charge. So that during calcining and fluxing portion of the melt the sulfur is captured by the slag and removed therewith. Thus a large portion of the energy produced by the coke is used to calcine the fluxing agent. The high quantity of fluxing agent also has a deleterious effect on the refractory lining, and also traps alloying elements such as silicon and manganese requiring that these be made up by adding ferro-silicon and ferro-manganese to the charge, which is expensive.
Desulfurization of iron can also be accomplished outside the cupola by adding calcium carbide to the molten iron in a ladle and bubbling nitrogen through the molten mass to effectuate mixing. The calcium sulfide formed combines with the unreacted calcium carbide to produce a doss which is skimmed from the top of the molten iron. The doss reacts with water to form acetylene which, when mixed with air, is explosive.
The object of this invention is to provide a method of desulfurizing iron in the cupola which is energy efficient, does not trap required alloying elements, does not produce an explosive by-product and is not deleterious to the refractory lining of the cupola.